The present invention relates to an improved non-clogging and wear-reducing vibratory screen and to an improved vibratory screening machine in which it can be used.
By way of background, in U.S. Pat. No. 4,575,421 there is disclosed a non-clogging wear-reducing vibratory screen wherein a coarse screen and a plurality of fine screens are bonded to an apertured plate which is vibrated in a vibratory screening machine. In this patent, a fine screen is mounted on top of a coarse supporting screen and a finer screen is mounted on top of the fine screen. The three layers of screens are bonded to the plate at the borders of the apertures. The foregoing screen operated satisfactorily in screening dry materials and liquid-solid mixtures. It has been found that when vibratory screening is effected with screens of the foregoing type under a head of fluid, that is, under a height of a liquid-solid mixture, more rapid screening can be effected than if the screening is being effected without such a head. However, it has been found that when the more rapid screening is being effected under such a head, the vibratory screens wear out more rapidly because the flexing produces fatigue failure of the screens at the borders of the apertures and because the fragile uppermost screen is exposed to abrasion by relatively large particles in the liquid-solid mixture.
Relative to the machine aspect of the present invention, kaolin is processed at its point of manufacture for use as a coating in the paper industry. The kaolin is ground so that it will be of a 325 mesh size, which is a size which passes through a 370 mesh screen. However, at the termination of processing it is in a 50% water solution. It is then in condition to be shipped to the paper manufacturer in railroad tank cars. However, for the sake of economy it is desired that the kaolin content be increased to 70%, rather than 50%, so that a transportation charge is not paid for shipping water. Therefore, dry powdered kaolin is added to the 50% kaolin solution to bring its concentration up to 70%. However, incidental to this thickening procedure, it is possible that larger sizes of kaolin will enter the mixture. Such larger sizes cannot be tolerated in the paper manufacturing process.
In the past, the 70% solution was therefore treated on vibratory screening machines to screen out any larger particles before it was loaded into a tank car. Such machines were capable of producing approximately five gallons per minute of the screened 70% solution. This was a relatively slow rate, and therefore numerous screening machines were operated simultaneously to fill a tank car in a reasonably short period of time.
In addition to the foregoing, in the oil drilling industry drilling mud is utilized during drilling, and this mud is screened to remove undesired large particles of foreign materials so that the expensive drilling mud can be reused. When the mixture of drilling mud and water and undesired large particles were screened with screens of the general type disclosed in U.S. Pat. No. 4,575,421, (1) the undesired large particles abraded the finest uppermost screen and thus produced tears therein after a relatively few hours of operation and (2) the head of the liquid-solid mixture caused fatigue failure of the open areas of the screen.